How are oxygen and carbon dioxide transported in the blood?

• A. Oxygen is carried only dissolved in plasma; carbon dioxide is carried as bicarbonate and carbaminohemoglobin.
• B. Oxygen is dissolved in plasma or binds to hemoglobin; carbon dioxide is dissolved as CO2, carbaminohemoglobin, or bicarbonate ions.
• C. Oxygen is carried as carbonate; carbon dioxide is carried only as dissolved CO2.
• D. Oxygen is carried as carbaminohemoglobin exclusively; carbon dioxide is carried only as bicarbonate.

Answer: (B)

Oxygen travels in the blood mainly by two forms: a small amount dissolved in plasma and most of it bound to hemoglobin within red blood cells, forming oxyhemoglobin; this dual mechanism lets blood carry enough O2 from lungs to tissues even though only a limited portion is dissolved directly in plasma. Carbon dioxide has three main pathways: a portion is dissolved in plasma, another portion binds to hemoglobin as carbaminohemoglobin, and the largest portion is converted to bicarbonate (HCO3−) inside red blood cells by carbonic anhydrase and then transported in plasma. The bicarbonate form is key for efficient CO2 transport from tissues to lungs, with the chloride shift helping maintain electrical balance as bicarbonate leaves the red cell.

So the best description is that oxygen is carried both dissolved and bound to hemoglobin, while carbon dioxide is carried dissolved, as carbaminohemoglobin, or as bicarbonate ions.